linux-kernel - Re: CVE-2024-26821: fs: relax mount

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Date: Thu, 23 May 2024 15:56:44 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Anthony Iliopoulos <ailiop@...e.com>
Cc: Christian Brauner <brauner@...nel.org>, cve@...nel.org,
	linux-kernel@...r.kernel.org, linux-cve-announce@...r.kernel.org
Subject: Re: CVE-2024-26821: fs: relax mount_setattr() permission checks

On Tue, May 14, 2024 at 02:49:39PM +0200, Anthony Iliopoulos wrote:
> On Wed, Apr 17, 2024 at 11:44:04AM +0200, Greg Kroah-Hartman wrote:
> > Description
> > ===========
> > 
> > In the Linux kernel, the following vulnerability has been resolved:
> > 
> > fs: relax mount_setattr() permission checks
> > 
> > When we added mount_setattr() I added additional checks compared to the
> > legacy do_reconfigure_mnt() and do_change_type() helpers used by regular
> > mount(2). If that mount had a parent then verify that the caller and the
> > mount namespace the mount is attached to match and if not make sure that
> > it's an anonymous mount.
> > 
> > The real rootfs falls into neither category. It is neither an anoymous
> > mount because it is obviously attached to the initial mount namespace
> > but it also obviously doesn't have a parent mount. So that means legacy
> > mount(2) allows changing mount properties on the real rootfs but
> > mount_setattr(2) blocks this. I never thought much about this but of
> > course someone on this planet of earth changes properties on the real
> > rootfs as can be seen in [1].
> > 
> > Since util-linux finally switched to the new mount api in 2.39 not so
> > long ago it also relies on mount_setattr() and that surfaced this issue
> > when Fedora 39 finally switched to it. Fix this.
> > 
> > The Linux kernel CVE team has assigned CVE-2024-26821 to this issue.
> 
> This one probably needs to be disputed as it isn't an actual
> vulnerability, but rather a fix for the mount_setattr which previously
> didn't allow reconfiguring the real rootfs similar to what the mount
> syscall always allowed to do.
> 
> So it merely brings mount_attr up to par with mount in terms of allowing
> the real rootfs to be reconfigured.

Yes, it fixes a problem where the system could not be booted
properly without this fix.

> Christian, what do you think ?

I'll defer to Christian if this CVE should be revoked or not.

thanks,

greg k-h